Titanium alkoxides are characterized in U.S. Pat. No. 3,379,654, and the Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 20, pp. 491-493, as cross-linking agents for epoxide resins. They form `titanated`, soluble, curable epoxide prepolymers for fibrous laminates, which can be further hardened with amines, amides, Friedel-Crafts catalysts, and polycarboxylic acid/anhydrides. In U.S. Pat. Nos. 2,742,448; 2,962,410; 3,123,582 and 3,385,835; alkyl-titanates were used as combination moderating and curing agents with amine curing or complexing agents, and as curing accelerators with polycarboxylic acid-anhydrides, for epoxide resins used as pre-impregnated insulation lay-ups, metal coating compositions, varnishes, casting resins, molding compositions and adhesives. None of the organo-titanates were used in combination with mica to form a latent-catalyst for flexible insulation pre-pregs.
In the manufacture of large rotating machines, epoxide resins have long been used as impregnants for insulation which relies upon mica, in the form of paper, flakes or large splittings as the dielectric. In this art, epoxide-polycarboxylic acid/anhydride systems are generally catalyzed with materials such as dicumyl peroxide and tertiary butylperbenzoate, as in U.S. Pat. No. 3,647,611, or with quaternary organic phosphonium compounds, as in U.S. Pat. No. 3,759,866. These catalyzed, epoxide-polycarboxylic acid/anhydride impregnants are then vacuum impregnated into prewound mica tape insulated coils.
Vacuum impregnation, however, is costly and time consuming, but has generally been considered necessary to get a void free insulating tape. Groff, in U.S. Pat. No. 3,660,220, used as a mica-glass cloth tape, pre-impregnated with a toluene-methyl isobutyl ketone solution of an epoxide-hydrogenated caster oil modified acid anhydride, as a pre-preg electrical insulation for motors. These solutions were catalyzed with stannous octoate, tertiary amines or boron trifluoride complexes. These catalysts, however, provide poor high temperature electrical properties, as shown by dissipation (power) factor values of 25% at 155.degree. C. Also, these mica tapes would not retain their initial flexibility after storage.
While the pre-preg tape concept is an improvement in the art, it presents a host of problems with respect to tensile strength, void-free resin impregnation, and retention of flexibility after long periods of storage i.e. over 6 months. What is needed then, is a new mica latent catalyzed resinous insulation tape for high voltage motors and large rotating machines, that is void-free and will retain flexibility after long term storage.